T cell proliferation is essential for effective immune responses against foreign Ags but must be tempered to prevent autoimmunity and preserve tolerance. E3 ubiquitin ligases are involved in balancing T cell activation and anergy. Lin et al. (p. 5919) have shown that naive T cell expression of one such enzyme, the gene related to anergy in lymphocytes (GRAIL), can inhibit IL-2R-mediated proliferation upon activation. Naive T cell costimulation with anti-CD3 and anti-CD28 resulted in GRAIL degradation, most likely mediated by the deubiquitinating enzyme Otubain-1 (Otub1). Subsequent induction of IL-2 production, IL-2R signaling, and activation of the mammalian target of rapamycin (mTOR) kinase resulted in T cell proliferation. The signaling pathway activated by mTOR induced Otub1 expression and concomitant GRAIL degradation. The mTOR pathway was inhibited with CTLA4-Ig treatment by blockage of CD28 engagement and IL-2 synthesis or with the addition of rapamycin or anti-IL-2 that hindered IL-2R-driven mTOR activation. Although GRAIL and Otub1 were first defined for their roles in T cell anergy, these results highlight their contribution to the signaling events linking T cell activation and proliferation.

Human immunodeficiency virus typically infects T cells by interactions mediated through the CD4 receptor and the CXCR4 or CCR5 coreceptors, but the virus can latently infect other cell types expressing these molecules, including FcεRI+ progenitor mast cells (prMCs). PrMCs developed in culture or isolated from peripheral blood are permissive to CCR5-tropic (R5) HIV infection but are less easily infected by CXCR4-tropic viruses (X4). However, X4 virus usually becomes the predominant variant in many HIV-infected individuals, and this shift predictably correlates with a hastier progression to AIDS. Sundstrom et al. (p. 6401) have identified a role for IgE-FcεRI interactions in augmenting the susceptibility of prMCs to X4 viruses, primarily by up-regulation of CXCR4 surface expression during mast cell ontogeny. Treatment of cultured prMCs with IgE specifically up-regulated CXCR4 gene expression and increased permissibility to X4 virus, whereas no such effect was seen for R5 virus. X4 virus was inhibited by the pretreatment of cells with peptides specific to CXCR4 or with omalizumab, a mAb that blocks IgE-FcεRI binding. Conversely, IgE- FcεRI aggregation induced with superallergens, including HIV gp120, enhanced X4 virus infection. These results indicate that elevated IgE levels, induced by allergy or infection, may contribute to a shift in HIV tropism, especially in cells harboring latent infections.

A major challenge to the discovery of effective vaccines for the elderly is the deterioration of adaptive immune responses during aging. Diminished functionality of aged Th cells has been identified as a key source of these age-related immune defects. Maue et al. (p. 6129) used proinflammatory adjuvants to restore helper functions to these T cells in a murine model of adoptive transfer and immunization. Aged Th cells could promote effective Ag-specific B cell responses in young or aged mice through vaccination with proinflammatory adjuvants, including poly(I:C), a TLR3 ligand, or a combination of the proinflammatory cytokines TNF-α, IL-1, and IL-6. These proinflammatory cytokines directly stimulated naive T cells from aged mice to produce cytokines associated with Th1, Th2, and Th17 responses. mRNA for the Th17-associated transcription factor RORγt was also robustly up-regulated in this cell population, indicating the development of a Th17 subset. T cells polarized into a Th17 phenotype in vitro from young or aged mice provided potent B cell help in vivo, and this finding provides insight into a potential T cell subset that could be targeted in vaccines developed for the elderly. These results demonstrate that proinflammatory adjuvants provide vigorous stimulation to both young and aged T cells, thus inducing the development of multiple Th cell subsets as well as effective humoral responses.

Lipid rafts are membrane microdomains that serve as signal transduction sites, but their role in lymphocyte signaling is poorly defined. Saeki et al. (p. 5929) identified Raftlin (raft-linking protein) as an abundant lipid raft-associated protein in lymphocytes, and they generated Raftlin-deficient and Raftlin-transgenic (Tg) mice that overexpress this protein in B and T cells to define a physiological role for Raftlin. Raftlin-deficient mice showed no defect in BCR signaling, although Raftlin 2, a Raftlin homologue, was highly expressed in B cells and may have compensated for the deficiency. In contrast, Ab and T cell responses were reduced in Raftlin-deficient mice immunized with a T-dependent Ag, while such responses were enhanced in Raftlin-Tg mice. Strikingly, experimental autoimmune encephalomyelitis (EAE) was attenuated in Raftlin-deficient mice compared with controls, whereas Raftlin-Tg mice developed severe EAE. Cytokine levels were different between these groups, and IL-17 production in particular was reduced in T cells from Raftlin-deficient mice but increased in Raftlin-Tg mice. A difference in TCR signal strength was identified as a mechanism behind these divergent phenotypes, and a TCR-associated kinase, Lck, was reduced in lipid rafts in the absence of Raftlin but increased by Raftlin overexpression. Thus, Raftlin appears to modulate TCR signaling, especially with respect to Th17 differentiation.

Acute vascular rejection (AVR) is a devastating complication associated with organ transplantation that is typically mitigated by immunosuppressive drug therapy. Induction of graft tolerance is the ideal approach for preventing rejection, and this has been achieved through highly risky hematopoietic stem cell transplantation. Wang et al. (p. 5970) have identified a new method for prolonging allograft survival in a mouse model of AVR by cardiac transplantation in the presence of a free bone graft. Subcutaneous transplantation of bone from allogeneic C3H donor mice or third party C57BL/6 mice into BALB/c recipients significantly prolonged the survival of heart grafts upon cotreatment with the immunosuppressive drug cyclosporine (CsA) alone. Mesenchymal stem cells (MSCs) in the bone graft were sufficient for inhibiting alloantibody development and preventing rejection. Unfortunately, MSCs did not induce a tolerogenic state, as withdrawal of CsA treatment resulted in rapid rejection of cardiac allografts. The authors also showed that donor MSCs regulated the maturation of recipient CD11c+ dendritic cells (DCs) and conferred graft survival in secondary allograft recipients treated with DCs from long-term surviving recipients and CsA. The innovative strategy presented here has potential for advancing organ transplantation protocols and improving the efficacy of immunosuppressive agents.

Natural killer cells respond to both pathogen and neoplastic assaults by lysing infected or cancerous cells and secreting IFN-γ. CD56dim NK cells are typically potent cytotoxic cells, while IFN-γ production has been observed in CD56bright NK cells. Korbel et al. (p. 6426) provide new evidence that IFN-γ is produced by both NK cell subsets in response to microbial or cytokine stimuli and that the level of IFN-γ production is dependent on the killer Ig-like receptor (KIR) genotype. Both CD56dim and CD56bright NK cells from human donors produced IFN-γ following in vitro exposure to various nonviral microbial ligands or accessory cell-derived cytokines. IFN-γ production between NK cell subsets from some donors was quite similar, whereas other donors showed large differences. The authors assessed the contribution of genetic factors to this variability by determining the genotype of the KIR locus. The cytokine response in CD56dim NK cells, measured by the ratio of IFN-γ-expressing CD56dim vs CD56bright NK cells, was significantly inhibited in KIR homozygotes as compared with heterozygotes. This result supported a link between cytokine production in CD56dim NK cells and KIR genotype, because KIR gene expression was highly variable in CD56dim NK cells and nearly absent in CD56bright NK cells. This study establishes the ability of NK cell subsets to produce IFN-γ in response to a variety of stimuli and suggests an influence of KIR genotype on the innate responsiveness of these cells.

Development of therapeutic peptides that use MHC class II presentation is fraught with challenges, as MHC class II molecules are not readily loaded with exogenous peptides. Call et al. (p. 6342) have identified small molecules that enhance peptide loading onto the human MHC class II complex HLA-DR (DR). Foreign peptides are typically loaded onto DR molecules residing in acidified endosomal compartments by interactions with HLA-DM (DM). The authors identified J10 as a small molecule that catalyzed accelerated peptide exchange in vitro without requiring DM. They developed analogues of J10 to further optimize this reaction, for which J10-1 showed 5-fold greater activity than J10. Both J10 and J10-1 were functional for several DR allotypes and different experimental peptides, and their activity could withstand a wide pH range. These compounds augmented APC presentation of exogenous peptides to T cells by increasing both the rate of peptide presentation and the abundance of peptide on the cell surface. Peptide presentation was also enhanced by J10-1 in vivo in DR-transgenic mice, and no nonspecific immune stimulation or toxicity was detected following treatment with this compound. The authors suggest that these compounds may be useful adjuncts to therapeutic peptide treatments and vaccines.

Neisseria gonorrhoeae infection is a widespread sexually transmitted disease that can have dire consequences for female fertility and neonatal health. Pathogenesis has been tied to the release of inflammatory mediators from innate immune cells sensing the presence of N. gonorrhoeae. Duncan et al. (p. 6460) found that inflammasome-mediated production of proinflammatory cytokines is dependent on N. gonorrhoeae detection by the Nod-like receptor (NLR) gene product NLRP3 in these cells. N. gonorrhoeae infection of primary monocytes or a monocyte-derived cell line induced the secretion of inflammatory cytokines, especially IL-1β and IL-18, but cytokine release was significantly reduced in monocytic cells lacking the inflammasome components NLRP3 or apoptotic speck protein containing a caspase recruitment domain (ASC). Caspase-1 activity was required for IL-1β secretion, whereas NLRP3-mediated pyronecrosis was deemed the likely mechanism of cell death and was exclusive of caspases. The cysteine proteinase cathepsin B was also activated by N. gonorrhoeae infection and was required for inflammasome activation upstream of NLRP3. Gonococcal lipooligosaccharide was able to activate NLRP3 signaling, suggesting at least one mechanism of N. gonorrhoeae-mediated inflammasome activation. These findings suggest that the striking inflammatory response caused by N. gonorrhoeae infection may be due in part to activation of NLRP3 signaling.

Summaries written by Christiana N. Fogg, Ph.D.